1. Field of the Invention
The present invention relates to a method for improving the cold flow of hydrocarbon fuel oils.
2. Description of the Prior Art
Since the oil crisis, a variety of sources for fuel oils have been used and the ratio of the amount of light crude oils has been reduced and it is therefore thought that the use of heavy crude oils will increase in the future. On the other hand, the demand for middle distillate fuel oils tends to be increased in view of the regulation of sulfur oxide exhaust. Therefore, if it is intended to obtain fuel oils from heavy crude oils containing a large amount of paraffins having high molecular weight through fractional distillation, it is necessary to remove the distillate fraction of high boiling points. As a result, the paraffin content having high molecular weight is increased in the distilled fuel oils.
In such fuel oils, crystals of paraffin are more apt to be precipitated and grown at a low temperature than in conventional fuel oils and the fluidity of the fuel oil is thereby lowered. Furthermore, large paraffin crystal grains form at a temperature at which the fluidity is maintained and filters in the fuel supply system and piping in diesel engines, etc. become plugged and the flowing of fuel oils is inhibited.
For solving these problems, a large number of cold flow improvers of fuel oils have been proposed, for example, condensation products of chlorinated paraffin and naphthalene (U.S. Pat. No. 1,815,022), polyacrylates (U.S. Pat. No. 2,604,453), polyethylenes (U.S. Pat. No. 3,474,157), copolymers of ethylene and propylene (French Pat. No. 1,438,656) and copolymers of ethylene and vinyl acetate (U.S. Pat. No. 3,048,479) and the like.
When these cold flow improvers are added to fuel oils, they show an excellent function for lowering the pour point in a pour point test (JIS K 2269) but in many cases have substantially no effect in a cold filter plugging point test (abbreviated as CFPP test hereinafter) by which the plugging of a filter in the fuel supply system at low temperatures is judged. The improvers which are effective with fuel oils containing a large amount of paraffin of high molecular weight, are few.
The pour point test cannot forecast the plugging of the filter in the fuel supply system due to paraffin crystal grains formed at a temperature higher than the pouring point but the CFPP test can serve to forecast this phenomenon and is presently widely used.
There has been proposed in EPC Laid-Open Specification No. 0085803 a cold flow improver capable of effectively lowering the CFPP of fuel oils. However, this cold flow improver has drawbacks, for instance the improver has a high melting point and is hardly soluble in fuel oils.